1. Field of the Invention
The present invention relates to an echo canceller used, for example, for a two-wire full-duplex data modem etc.
In a full-duplex communication system using the above-mentioned two-wire full-duplex data modem, a hybrid circuit is adopted for the two wire-four wire conversion in the data modem and exchange. At the portion where this hybrid circuit is provided, as mentioned later, a near-end echo and far end echo return to the transmission side modem and interfere with normal communication. Am echo canceller is used to eliminate this interference with communication.
2. Description of the Related Art
One of the technical problems in conventional echo cancellers has been the frequency offset, explained in detail later. This is a problem unique to frequency division multiplex (FDM) type communication systems. If the echo is cancelled with consideration given to this frequency offset too, a high quality of echo cancellation can be realized. Therefore, in the past, a frequency-offset correction circuit has been built into the echo cancellers.
However, the echo cancellers in the prior art cannot correct the frequency offset for all far end echo characteristics. From this viewpoint, there is a first problem in that an echo canceller with a sufficiently good quality cannot be realized.
Further an echo canceller must process two echoes, the near-end echo and far-end echo, separately, but in actuality since the two echoes return to the transmission side modem simultaneously with the low level far end echo buried in the high level near end echo, there is a second problem that the two are difficult to distinguish. This second problem makes the resolution of the above mentioned first problem increasingly difficult. The reason for this is that the above-mentioned first problem is caused due to only the far-end echo and has nothing at all to do with the near-end echo.